In recent years, in particular, reduction of fuel consumption of vehicles is highly required in consideration of environment. Tires affect the fuel efficiency of vehicles, and therefore, a “low fuel consumption tire” that contributes to reduction in fuel consumption is under development.
In order to reduce fuel consumption by means of tires, it is important to reduce the rolling resistance of the tires. Among the factors of generation of the rolling resistance of a tire are an energy loss caused by the deformation of the tire during rolling, an energy loss caused by the friction between the tire and the road surface, and the like. Of the parts of a tire, these losses are largest in a tread. For typical tires for passenger cars, the losses at the tread account for about 40% of the overall losses. Decrease in the losses at the tread may contribute significantly to reduction in rolling resistance. In order to reduce an energy loss at the tread, it is necessary to minimize the deformation of the tread of a tire during rolling.
A pneumatic tire includes a belt for reinforcing a carcass. The belt typically includes an inner layer and an outer layer. FIG. 6 shows a part of a belt 2 including an inner layer 4 and an outer layer 6. In FIG. 6, the up-down direction represents the circumferential direction of the tire, the left-right direction represents the axial direction of the tire, and the direction perpendicular to the drawing sheet represents the radial direction of the tire. In FIG. 6, an alternate long and short dash line CL represents an equator plane of the tire.
The inner layer 4 and the outer layer 6 each include multiple cords 8 aligned with each other, and a topping rubber 10. Each cord 8 is inclined relative to the equator plane of the tire. As shown in FIG. 6, the inclination angle of each cord 8 in the inner layer 4 relative to the equator plane and the inclination angle of each cord 8 in the outer layer 6 relative to the equator plane, represent values in opposite directions, respectively. The absolute value of the inclination angle of each cord 8 in the inner layer 4 relative to the equator plane is equal to the absolute value of the inclination angle of each cord 8 in the outer layer 6 relative to the equator plane. In the description herein, the absolute value is represented by α. Typically, the absolute value α is small. Typically, the absolute value α is less than or equal to 35°. The main reasons therefor may be as follows.
(1) The stiffness of the tread in the circumferential direction can be increased by decreasing the absolute value α. As a result, the deformation of the tread during rolling of the tire is reduced.
(2) It is known that the profile of a tread of a tire having a small absolute value α is flatter than the profile of a tread of a tire having a large absolute value α. Based thereon, the deformation of the tread during rolling of the tire can be reduced.
(3) Increase of the absolute value α leads to decrease of the stiffness of the tread in the circumferential direction. Therefore, when the tire is inflated with air, the “projection” of the tread is likely to occur. This may lead to occurrence of a crack in the groove bottom of the tread (referred to as a tread groove crack, which is hereinafter represented as TGC).
As described above, conventionally, it is generally believed that if the absolute values α of the inclination angles of the cords in the inner layer and the cords in the outer layer relative to the equator plane are decreased, the deformation of the tread is reduced, so that the rolling resistance of the tire can be reduced. Moreover, the reduction in the absolute value α also contributes to reduction in the occurrence of a TGC. An exemplary tire having a belt in which the absolute value α is small is disclosed in JP2013-107518.